1. Field of the Invention
This invention relates to a process for preparing a polyester-modified silicone resin by a simple preparation procedure. The invention also relates to a curable silicone resin composition which comprises a polyester-modified silicone resin of the type mentioned above and also to a cured product obtained therefrom.
2. Description of the Prior Art
As is known in the art, polyester-modified silicone resins have good weatherability and heat resistance and also have such good characteristics as to suffer only a reduced degree of discoloration through thermal degradation on comparison with those of other types of modified silicone varnishes. Thus, the polyester-modified silicone resins have wide utility in the fields of paints or varnishes for buildings, automobiles, mufflers for transport planes, cooking utensils and the like.
In usual practice, polyester-modified silicone resins have been prepared by initially preparing polyester resins from saturated polybasic acids and polyhydric alcohols, and then preparing silicone resins by hydrolysis and condensation of chlorosilane or alkoxysilanes as having a molecular weight of about 1,000. Subsequently, such silicone resins are subjected to dehydration or de-alcoholization condensation reaction with the polyester resins in the presence of a metallic catalyst, such as a titanium catalyst, thereby obtaining the modified silicone resins.
However, the above process broadly comprises the three steps including the step of preparing a polyester resin, the step of preparing a silicone resin, and the step of the modification. Thus, the process is complicated in its procedure.
Moreover, the resultant polyester-modified silicone resins have to be cured under high temperature and long-term heating conditions or by use of curing catalysts, such as a lead catalyst, in order to obtain a cured film having optimum hardness and wear resistance. The high temperature and long-term heating conditions require high costs, and the use of the lead catalyst presents a problem on safety.
The cured film obtained from polyester-modified silicone resins may suffer yellowing owing to its low heat resistance. To avoid this, it is known to add zinc compounds to the silicone resins, by which the heat resistance and yellowing deficiency can be improved to an extent (Japanese Laid-open Patent Application No. 5-255638). However, the zinc compounds have no catalytic action on the curing reaction. For polyester-modified silicone resins prepared according to known processes, it is essential to use curing catalysts such as of lead in order to obtain high curability.